Appender UNION support (1.4)

This is a backport of the PR #372 to `v1.4-andium` stable branch.

This PR adds support for appending data to `UNION` columns. The tag
(field name) of the union must be specified in `beginUnion()` call
before appending the actual value.

Nested unions (or structs inside unions) are not supported.

Example:

```sql
CREATE TABLE tab1 (col1 INTEGER, col2 UNION(u1 INTEGER, u2 VARCHAR))
```

```java
DuckDBAppender appender = conn.createAppender("tab1");

// append row with INTEGER value in UNION
appender
    .beginRow()
    .append(1)
    .beginUnion("u1")
    .append(42)
    .endUnion()
    .endRow();

// append row with VARCHAR value in UNION
appender
    .beginRow()
    .append(2)
    .beginUnion("u2")
    .append("foo")
    .endUnion()
    .endRow();
```

Testing: existing test is extended to cover `UNION`s.

Author: staticlibs

duckdb_java.def

@@ -58,6 +58,7 @@ Java_org_duckdb_DuckDBBindings_duckdb_1create_1list_1type
 Java_org_duckdb_DuckDBBindings_duckdb_1create_1array_1type
 Java_org_duckdb_DuckDBBindings_duckdb_1create_1struct_1type
 Java_org_duckdb_DuckDBBindings_duckdb_1struct_1type_1child_1count
+Java_org_duckdb_DuckDBBindings_duckdb_1struct_1type_1child_1name
 Java_org_duckdb_DuckDBBindings_duckdb_1array_1type_1array_1size
 Java_org_duckdb_DuckDBBindings_duckdb_1destroy_1logical_1type
 Java_org_duckdb_DuckDBBindings_duckdb_1create_1vector

duckdb_java.exp

@@ -55,6 +55,7 @@ _Java_org_duckdb_DuckDBBindings_duckdb_1create_1list_1type
 _Java_org_duckdb_DuckDBBindings_duckdb_1create_1array_1type
 _Java_org_duckdb_DuckDBBindings_duckdb_1create_1struct_1type
 _Java_org_duckdb_DuckDBBindings_duckdb_1struct_1type_1child_1count
+_Java_org_duckdb_DuckDBBindings_duckdb_1struct_1type_1child_1name
 _Java_org_duckdb_DuckDBBindings_duckdb_1array_1type_1array_1size
 _Java_org_duckdb_DuckDBBindings_duckdb_1destroy_1logical_1type
 _Java_org_duckdb_DuckDBBindings_duckdb_1create_1vector

duckdb_java.map

@@ -57,6 +57,7 @@ DUCKDB_JAVA {
     Java_org_duckdb_DuckDBBindings_duckdb_1create_1array_1type;
     Java_org_duckdb_DuckDBBindings_duckdb_1create_1struct_1type;
     Java_org_duckdb_DuckDBBindings_duckdb_1struct_1type_1child_1count;
+    Java_org_duckdb_DuckDBBindings_duckdb_1struct_1type_1child_1name;
     Java_org_duckdb_DuckDBBindings_duckdb_1array_1type_1array_1size;
     Java_org_duckdb_DuckDBBindings_duckdb_1destroy_1logical_1type;
     Java_org_duckdb_DuckDBBindings_duckdb_1create_1vector;

src/jni/bindings_logical_type.cpp

@@ -2,6 +2,7 @@
 #include "refs.hpp"
 #include "util.hpp"
 
+#include <cstring>
 #include <vector>
 
 duckdb_logical_type logical_type_buf_to_logical_type(JNIEnv *env, jobject logical_type_buf) {
@@ -209,8 +210,9 @@ JNIEXPORT jobject JNICALL Java_org_duckdb_DuckDBBindings_duckdb_1create_1struct_
 		if (env->ExceptionCheck()) {
 			return nullptr;
 		}
-		names_cstr_vec.emplace_back(str.c_str());
 		names_vec.emplace_back(std::move(str));
+		std::string &str_ref = names_vec.back();
+		names_cstr_vec.emplace_back(str_ref.c_str());
 	}
 
 	duckdb_logical_type struct_type =
@@ -237,6 +239,40 @@ JNIEXPORT jlong JNICALL Java_org_duckdb_DuckDBBindings_duckdb_1struct_1type_1chi
 	return static_cast<jlong>(count);
 }
 
+/*
+ * Class:     org_duckdb_DuckDBBindings
+ * Method:    duckdb_struct_type_child_name
+ * Signature: (Ljava/nio/ByteBuffer;J)[B
+ */
+JNIEXPORT jbyteArray JNICALL Java_org_duckdb_DuckDBBindings_duckdb_1struct_1type_1child_1name(JNIEnv *env, jclass,
+                                                                                              jobject logical_type,
+                                                                                              jlong index) {
+
+	duckdb_logical_type lt = logical_type_buf_to_logical_type(env, logical_type);
+	if (env->ExceptionCheck()) {
+		return nullptr;
+	}
+	idx_t index_idx = jlong_to_idx(env, index);
+	if (env->ExceptionCheck()) {
+		return nullptr;
+	}
+
+	idx_t count = duckdb_struct_type_child_count(lt);
+	if (index_idx >= count) {
+		env->ThrowNew(J_SQLException, "Invalid struct field index specified");
+		return nullptr;
+	}
+
+	auto name_ptr = varchar_ptr(duckdb_struct_type_child_name(lt, index_idx), varchar_deleter);
+	if (name_ptr.get() == nullptr) {
+		return nullptr;
+	}
+
+	idx_t len = static_cast<idx_t>(std::strlen(name_ptr.get()));
+
+	return make_jbyteArray(env, name_ptr.get(), len);
+}
+
 /*
  * Class:     org_duckdb_DuckDBBindings
  * Method:    duckdb_array_type_array_size

src/jni/util.hpp

@@ -9,10 +9,14 @@ extern "C" {
 #include <memory>
 #include <string>
 
-using jstring_ptr = std::unique_ptr<const char, std::function<void(const char *)>>;
-
 using jbyteArray_ptr = std::unique_ptr<char, std::function<void(char *)>>;
 
+using varchar_ptr = std::unique_ptr<char, void (*)(char *)>;
+
+inline void varchar_deleter(char *val) {
+	duckdb_free(val);
+}
+
 void check_java_exception_and_rethrow(JNIEnv *env);
 
 std::string jbyteArray_to_string(JNIEnv *env, jbyteArray ba_j);